1. Field of the Invention
The present invention relates to an oxygen concentration sensor used for air-fuel ratio control of an automobile engine.
2. Description of Related Art
A conventional oxygen concentration sensor, which is disclosed in U.S. Pat. No. 5,039,972 (corresponding to JPU 2-146363) and shown in FIGS. 15 and 16, includes a housing 10, a plate sensor element 12 inserted in housing 10, and a ceramic holding member 9 having a rectangular holding hole 90 for supporting plate sensor element 12 and disposed between housing 10 and plate sensor element 12.
Powder 13 filled and pressurized by a pressurizing member 14 placed above of housing 10 is disposed on ceramic holding member 9.
As shown in FIG. 16, ceramic holding member 9 has a tubular main body and holding hole 90 for holding plate sensor element 12. The circumference of an upper opening end 91 of holding hole 90 is a horizontal surface 93 and a tapered surface 92 is disposed around horizontal plane 93. As shown in FIG. 15, both horizontal plane 93 and conical surface 92 receive powder 13.
As shown in FIG. 15, however, at the time of filling and pressurizing powder 13 in a mounting process of plate sensor element 12, an impact is applied to the direction of arrows B around a boundary contacting plate sensor element 12, ceramic holding member 9 and powder 13, causing cracking of plate sensor element 12. The cracking of the element occurs more often when the impact is applied to a shorter (thinner) side of plate sensor element 12 as shown by an arrow T in FIG. 2B.
Therefore, the pressurizing speed has to be slowed down in order to prevent the cracking of the element in a conventional method. Although excessively large impact on plate sensor element 12 is avoided by this method, it takes a long time to mount plate sensor element 12. Controlling the pressurizing speed makes the mounting process of plate sensor element 12 more complicated. Furthermore, the conventional method is still ineffective in preventing force applied in the direction indicated by the arrow B to plate sensor element 12. Thus, the cracking of the element cannot be prevented completely.